Not Applicable.
Not Applicable.
1. Field of the Invention
The present invention relates to electrohydraulic systems for operating machinery, and in particular to control algorithms for such systems.
2. Description of the Related Art
A wide variety of machines have moveable members which are operated by an hydraulic actuator, such as a cylinder and piston arrangement, that is controlled by a hydraulic valve. Traditionally the hydraulic valve was manually operated by the machine operator. There is a present trend away from manually operated hydraulic valves toward electrical controls and the use of solenoid operated valves. This type of control simplifies the hydraulic plumbing as the control valves do not have to be located near an operator station, but can be located adjacent the actuator being controlled. This change in technology also facilitates sophisticated computerized control of the machine functions.
Application of pressurized hydraulic fluid from a pump to the actuator can be controlled by a proportional solenoid operated spool valve that is well known for controlling the flow of hydraulic fluid. Such a valve employs an electromagnetic coil which moves an armature connected to the spool that controls the flow of fluid through the valve. The amount that the valve opens is directly related to the magnitude of electric current applied to the electromagnetic coil, thereby enabling proportional control of the hydraulic fluid flow. Either the armature or the spool is spring loaded to close the valve when electric current is removed from the solenoid coil. Alternatively a second electromagnetic coil and armature is provided to move the spool in the opposite direction.
When an operator desires to move a member on the machine a joystick is operated to produce an electrical signal indicative of the direction and desired rate at which the corresponding hydraulic actuator is to move. The faster the actuator is desired to move the farther the joystick is moved from its neutral position. A control circuit receives a joystick signal and responds by producing a signal to open the associated valve. A solenoid moves the spool valve to supply pressurized fluid through an inlet orifice to the cylinder chamber on one side of the piston and to allow fluid being forced from the opposite cylinder chamber to drain through an outlet orifice to a reservoir, or tank. A hydromechanical pressure compensator maintains a nominal pressure (margin) across the inlet orifice portion of the spool valve. By varying the degree to which the inlet orifice is opened (i.e. by changing its valve coefficient), the rate of flow into the cylinder chamber can be varied, thereby moving the piston at proportionally different speeds. The electrical signal for the solenoid defines how much the inlet orifice is to open and thus the flow rate of hydraulic fluid through the valve to the actuator. Previous control circuits converted the joystick signal into a desired flow rate signal. This flow rate signal was then used to define the amount of electric current to apply to the valve""s solenoid to achieve a desired inlet orifice valve coefficient. Thus prior control algorithms were based primarily on inlet orifice metering using an external hydromechanical pressure compensator.
Recently a set of proportional solenoid operated pilot valves has been developed to control fluid flow to and from the hydraulic actuator, as described in U.S. Pat. No. 5,878,647. In these valves, the solenoid armature acts on a pilot poppet that controls the flow of fluid through a pilot passage in a main valve poppet. The armature is spring loaded to close the valve when electric current is removed from the solenoid coil.
The control of an entire machine, such as agricultural tractor or construction apparatus is complicated by the need to control multiple functions simultaneously. For example, control of a backhoe often requires that hydraulic actuators for the boom, arm and bucket must be simultaneously operated. In some cases, the aggregate amount of hydraulic fluid flow being demanded by the simultaneously operating functions exceeds the maximum flow that the pump is capable of producing. At such times, it is desirable that the control system allocate the available hydraulic fluid among those functions in an equitable manner, so that one function does not consume a disproportionate amount of the available hydraulic fluid flow.
A typical hydraulic system has a supply line that carries fluid from a source, a return line which carries fluid back to a tank, and a plurality of hydraulic actuators coupled to the supply line and the return line by a plurality of valve assemblies. The present method relates to controlling the valve assemblies to apportion the flow of fluid from a source equitably among the hydraulic actuators and maintain the velocity relationship among the actuators as desired by the operator.
The method involves requesting a desired velocity for a given one of the plurality of hydraulic actuators and then determining a required fluid flow based the metering mode and the desired velocity for the given hydraulic actuator. The aforementioned steps are performed for other ones of the plurality of hydraulic actuators, thereby producing a plurality of required fluid flows. The plurality of required fluid flows are summed to produce an aggregate required fluid flow for the hydraulic system.
An amount of fluid flow that is available from the source is determined. A proportion value is derived which represents a relationship between the aggregate required fluid flow and the amount of fluid flow available from the source. In the preferred embodiment, the proportion value represents how much of the required fluid flow can be provided by the amount of fluid flow available from the source. For example, the proportion is the percentage of the aggregate required fluid flow that can be satisfied by the flow available from all flow sources in the hydraulic system. Then a velocity command is calculated for each of the plurality of hydraulic actuators, wherein each velocity command is derived based on a respective desired velocity and the proportion value. The flow of fluid to each of the plurality of hydraulic actuators then is controlled in response to plurality of velocity commands.